Solving a century's long concrete shrinkage cracking problem has been an elusive goal for cement technologists. When Portland cement formulations are mixed with water, they immediately begin a hydration reaction of powder with moisture evolving heat, cement matrix formation and curing that can take up to 28 days to form a 90+% cured concrete or mortar. During this curing process, the hydrated cement experiences shrinkage which often times lead to shrinkage cracking that can sometimes be minimal or other times prove catastrophic. This is especially critical when structural concrete is utilized for the construction of dams, waterways, water containment and treatment facilities, bridges, parking garages, stadiums, high rise buildings, etc. If cracks emerge, water (especially with deicing or marine salts) can penetrate and potentially cause premature corrosion of steel reinforcing bars, and if water leaks out of structures such as dams and waterways, there is a loss of efficiency and service life, and in the worst cases failures that can be catastrophic.
Magnesium oxides have been used in the field for improving crack resistance of portland cement concretes and mortars. An example of this is use is described in Du, Chonghang, Concrete International, December 2005, p. 45. by the Chinese. They used a lightly burnt type (<1200° C. burning temperature) of Magnesium Oxide for many concrete Dam projects throughout China in the late 1900's into early 2,000's. The use in dams was with low cementitious contents of about 180 to 220 kg/m3. Typical structural concretes will have over 300 kg/m3 of cementitious materials. Zhibin, Z. et al, SP-262-30, p. 395. performed some work with a high dosage of shrinkage reduction admixtures (“SRA”) with cementitious content of 2% (by mass of cement) and 3% MgO. Though better results were obtained than by using SRA alone, wet expansion was high (greater than 0.1%) for the combinations. This could present a problem in constant wet storage. In addition, the SRA tested contained a siloxane, increasing the cost of the system.
Today, several concrete admixtures have been tried or used with some success. Materials previously tried are superplasticizers, expansion agents (calcium oxide, or expansive cements), shrinkage reduction admixtures usually glycol based products, different type pozzolans such as fly ash to partially replace the portland cement, and many other approaches.
A combination of MgO and CaO is discussed by Miao, C. et al, International RILEM Conference on Use of Superabsorbent Polymers and Other New Additives in Concrete, 15-18 Aug. 2010. They required approximately 10% addition of cementitious material to achieve good results. Furthermore, while early expansion was good, drying shrinkage still occurred. The use of CaO with a SRA is discussed by Maltese, C. et al, Cement and Concrete Research 35 (2005), p. 2244. They showed a decrease in shrinkage with combinations of CaO expansive agent and a SRA. The CaO used had 2% material retained on an 80-micron mesh, indicating that the CaO particles are larger than the cement particles. This will lead to unsightly CaO particles being visible. A finer composition, at the size of smaller cement particles, is too reactive, making its use not viable.
The use of SRA, based on various polymeric glycols, have been practiced for the last three plus decades to reduce the risk of shrinkage cracking of many concrete structures The believed mechanism by which SRA's operate is that when excess water begins to evaporate from the concrete's surface after placing, compacting, finishing and curing; an air/water interface or “meniscus” is set up within the capillaries or pores of the cement paste of the concrete. Because water has a very high surface tension, this causes a stress to be exerted on the internal walls of the capillaries or pores where the meniscus has formed. This stress is in the form of an inward pulling force that tends to close up the capillary or pore. Thus the volume of the capillary is reduced leading to shrinkage of the cement paste around the aggregates, leading to an overall reduction in volume. SRA's therefore, are believed to operate by interfering with the surface chemistry of the air/water interface within the capillary or pore, reducing surface tension effects and consequently reducing the shrinkage as water evaporates from within the concrete. It has also been reported by others, that SRA's might mitigate plastic and autogenous volume changes. SRA's are relatively expensive, so their usage levels in the field are generally at or below a 2% level, based on the cement binder concentration. Higher concentrations provide only marginally less shrinkage, not justifying the higher costs, and often lead to excess retardation (increased time for the concrete to harden and develop strength) which is unacceptable. Performance is almost linear up to 2%, but under demanding situations, the low addition rates will not provide enough shrinkage reduction to prevent cracking. In addition to the patents listed, there are several articles in the literature on the effectiveness of SRAs in reducing shrinkage in cementitious systems. A good overview was given by Sant, G. et al, International RILEM Conference on Use of Superabsorbent Polymers and Other New Additives in Concrete, 15-18 Aug. 2010.
Jensen, O. and Hansen, P. F., Cement and Concrete Research, Vol. 31, No. 4 (2001), p. 647; Igarashi, S. et al, International RILEM Conference on Use of Superabsorbent Polymers and Other New Additives in Concrete, 15-18 Aug. 2010; Ribero, A. et al, International RILEM Conference on Use of Superabsorbent Polymers and Other New Additives in Concrete, 15-18 Aug. 2010; and Craeye, B. et al, Construction and Building Materials, 25 (2011), p. 1, represent several of the many references on the performance of Super Absorbent Polymers (“SAP”) when used in concrete formulations. These articles show that SAPs are effective in controlling internal desiccation of cementitious materials with low water to cement ratios, that is reduce autogenous shrinkage. These SAP materials are very expensive (compared to other concrete additives) and are cost prohibitive when used at the levels found to be effective earlier. The cited SAP's are based on various polyacrylics and polyacrylamides, mono or co-polymers. Other known SAP's are based on various cellulosics, fiber based materials, starches, polyacrylonitrile, polyvinyl alcohols, carboxymethyl cellulose, and isobutylene maleic anhydride.
SAPs provide additional water to balance the water lost to hydration of the cementitious components that can't be replaced from external water due to the low permeability of these materials. At higher water contents the need for extra water is less and these materials could potentially pull water out of the matrix. In addition, they do not provide enough water to offset moisture loss at higher permeability.
A novel and synergistic approach of blending shrinkage reduction admixtures (“SRAs”) with light-burnt and reactive magnesium oxide as expansion additives, along with the use of various super absorbent polymers; for improving many types of portland cement based concretes and mortars is disclosed. Superplasticizers are also found to be helpful when water reducing properties are required. MgO is less reactive than CaO and thus can be used in smaller particle sizes that do not adversely affect early setting or appearance. In addition, an unexpected synergy was found when SAPs were added to the combination of MgO and SRA. The combination synergies result in good shrinkage performance when the MgO level is at or less than 3% by mass of cementitious material and the SRA is less than 1.5% by mass of the cementitious materials present. This reduces the risk of expansion stresses that can cause cracking, and reduces the cost of a higher 2% or more dosage of SRA, as well as a reduction in strength of the admixture at higher SRA doses.
It is known that a liquid can be absorbed onto fine particles to have a free flowing dry powder. In the case of applying SRA to MgO ratios are needed that will result in a maximum dosage of about 6% MgO and a range of SRA from about 0.5 to 2% of the cementitious admixture by mass on cementitious. This requires a SRA percentage of the MgO from about 10 to 30%. The MgO has an average particle size of approximately 18 μm, making it on the order of some coarser cements. Cements will not absorb more than 3-5% of the SRA and still be flowable powder and not partially like a liquid. U.S. Pat. No. 6,648,962 shows that a hydrated cement can be crushed and act as a carrier for an SRA. But, this is a commercially more costly route, and unlike the MgO it offers no synergies on shrinkage/crack reduction. Other materials with high surface area, such as silica fume or metakaolin, can absorb the SRA, but offset some of the benefits of the SRA, and the combinations can significantly reduce workability of the mortar or concrete. Silica fume has another negative in that it darkens the mortar or concrete, which adversely affects its appearance. Expansive cements could potentially be used, but they typically require dosages of about 12% cement and require substantially more processing and cost to produce than the MgO used here. CaO can potentially work, but in the particle size range needed to prevent rapid expansion and heat generation, the light colored particles are highly visible on the surface of the mortar or concrete.
In this invention novel combinations of MgO with a liquid SRA to produce a stable flowing powder that allows for the application of the MgO and SRA together without the need for a liquid dispenser for the SRA. Optimum ratios for performance in cementitious systems as well as product stability were determined.